1. Field of the Invention
This invention relates to large Stokes shift fluorescent materials which fluoresce in the visible radiation spectrum. More particularly, this invention relates to improved organic scintillator systems useful for detecting high energy particles and electromagnetic radiation.
2. Description of Related Art
Bis-benzazoles connected through a central phenylene group have been used as fluorescent whitening or brightening agents, for instance as disclosed in U.S. Pat. Nos. 2,737,516; 3,293,258 and 3,575,996.In particular, bis-benzoxazoles are disclosed containing a central phenylene group which may be substituted with lower alkyl groups or halogen atoms.
U.S. Pat. No. 3,314,894 discloses the use of bis-benzoxazolyl compounds in scintillators. This patent discloses that the compound must be free of ionizable or dissociable units, such as --OH groups or the --NH groups of imidazole compounds.
Bis-benzazolyl hydroquinones and their alkoxy, alkylcarbonyl, lower alkyloxycarbonyl, benzoyl and phenoxycarbonyl derivatives are disclosed in Orlando et al., U.S. Pat. No. 3,673,202 in which it is disclosed that the compounds fluoresce in the visible and near-infrared regions of the spectrum when exposed to ultraviolet light. The compounds are photochemically and thermally stable and can be dissolved in organic solvents. Referring to the hydrogen substitution on the hydroquinone moiety, i.e., "R", Orlando et al. indicate that when the R substituents are other than hydrogen, they increase the solubility of the compounds in organic solvents The compounds can be incorporated in various polymers which can be used to produce films, molded objects, or applied as coatings on ultraviolet lamps to produce various colored lights when energized. In a publication (Chemical Communications, 1971, Pages 1551-1552) entitled "Red-and Near-infrared-luminescent Benzazole Derivatives", Orlando et al. compare the luminescent properties of 2-(2-hydroxyphenyl)benzazoles to those of bis-2,5-(2-benzazolyl)hydroquinones and their methyl ether derivatives wherein the benzxazole moiety is benzthiazole, benzoxazole, and benzimidazole. The ultraviolet spectra of the bis-benzazolehydroquinones in dimethylformamide was reported to have intense absorption from 320 to 410 nm (epsilon 26,000 to 43,000). From luminescence determined in the solid state, they observed that ultraviolet stimulated luminescence of the bis unsubstituted compounds was in the red and near infrared, and when substituted with methyl a hypsochromic shift of the emission band occurred. In comparison, they noted that the unsubstituted hydroxyphenylbenzazoles emitted in the blue end of the visible region and that the replacement of the o-hydroxy group by an o-methoxy group renders these benzazoles non- luminescent. In neither reference did Orlando et al. report or suggest that the bis-2,5-(2-benzazolyl)hydroquinones are soluble in styrene, vinyltoluene, xylene, or other such high efficiency scintillator solvents.
Mordzinski et al. have investigated excited-state proton-transfer reactions in 2-(2'-hydroxyphenyl)benzoxazole (J.Phys.Chem., 1986, 90, 5503-5506), in 2,5-bis(2-benzoxazolyl)hydroquinone (Chemical Physics Letters 1984, 111, No. 4,5; 383-388), and 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (J.Phys.Chem. 1986, 90, 1455-1458). In the latter study, Mordzinski et al. indicates that 2,5-bis(2-benzoxazolyl)hydroquinone dissolved in 2-methyl tetrahydrofurane has an extinction coefficient of about 20,000 at about 24,000 cm.sup.-1 (417 nm) and that 2,5-bis(2-benzoxazolyl)-4-methoxyphenol has an extinction coefficient of about 25,000 at about 27,000 cm.sup.-1 (370 nm). Also in the latter study, Mordzinski et al. reported that 2,5-bis(2-benzoxazolyl)-4-methoxyphenol exhibits dual luminescence arising from primarily excited and proton-transferred species and that from temperature studies of relative fluorescent quantum yields (from 300.degree. to 12.degree. K.), they concluded that excited-state intramolecular proton transfer (hereinafter identified as ESIPT) was found to occur effectively at 77.degree. K.
A number of plastic scintillators have been developed for the detection of high energy particles and radiation. Such plastic scintillators typically are comprised of a polymeric matrix, e.g., poly(vinyltoluene) (PVT) and a fluor (fluorescent compound), e.g., 3-hydroxy flavone (3-HF). Portions of such scintillators are expected to withstand radiation levels in excess of 10.sup.4 to 10.sup.5 Gy/yr without degradation of scintillator performance. Standard commercial plastic scintillators are known to suffer significant changes in performance at such radiation levels, typically due to formation of yellow to brown coloration in the matrix which absorbs substantial luminescence in the blue to UV spectral region. Attempts to restore the performance of radiation discolored plastic scintillators by treatments, such as by annealing or with oxygen, have only met with limited success. The issue of "radiation hardness" of plastic scintillators was the subject of a workshop on Mar. 19-21, 1990. (See Proceedings of the Workshop on Radiation Hardness of Plastic Scintillator; Mar. 19-20, 1990; Florida State University, Tallahassee, Fla.; Editor, Kurtis F. Johnson.) In these Proceedings, Clough et al., Pages 15-28, discusses radiation effects on scintillating fiber optics for the Super-conducting Super Collider (SSC); Zorn, Pages 1-14 discusses the design of a radiation-hard plastic scintillator for high luminosity HADRON colliders; and Kasha et al., Pages 49-60, discusses the molecular electronic criteria for the selection of radiation-hard scintillators each of which is included herein by reference. U.S. Pat. No. 4,594,179 discloses a method of reducing reabsorption effects in scintillators by using solutes with large Stokes shifts, i.e., a large shift towards the red in the emitted luminescence from the region of the absorbed radiation. 3-hydroxy flavone (3-HF) is proposed as such a solute in a scintillator matrix such as poly(vinyltoluene) (PVT).
Although advances have been made to produce a radiation hard scintillator, there still exists a need to produce radiation hard fluors which are characterized by: a very high extinction coefficient in the near ultraviolet; a Stokes shifted fluorescence emission which is free of phosphorescence emission and is spectrally matched to the sensitivity of photodetection systems used; a high fluorescence efficiency at room temperature; a short fluorescence lifetime to provide fast system response times; and substantially no self-absorption of the emitted fluorescence by the fluor. Moreover, there still exists a need for a fluor that is soluble in the polymeric matrix as well as the monomer precursors thereto; is substantially stable during addition polymerization of the polymeric matrix and during prolonged exposure to high energy radiation; and is photochemically stable and resistant to ambient oxidation.